1. Field of the Invention
The present invention relates to a method of controlling base stations, and more particularly, to a method of controlling base stations that is capable of suppressing inter-cell interference.
2. Description of Related Art
In a conventional frequency division multiple access (“FDMA”)/time division multiple access (“TDMA”) based cellular system, adjacent cells do not share resources for avoidance of interference. This allows a sufficient signal to interference ratio while low frequency reuse efficiency of the system results in reduced system capacity.
In the prior art FDMA/TDMA systems, voice service having a substantially constant data rate is mainly provided. To expand the system capacity, it is required to increase the number of available channels having a sufficient signal to interference ratio through power control. Code division multiple access (“CDMA”) based cellular systems have spread since they are capable of significantly increasing the frequency reuse efficiency. CDMA systems reduce a variation in interference with each channel through interference averaging, allowing most of channels to suffer moderate interference for a voice call.
This method, however, is no longer optimal as primary service has shifted from voice service of constant data rate to packet service of variable data rate. In addition, an issue of addressing interference between adjacent cells has been raised as an orthogonal frequency division multiplexing/orthogonal frequency division multiple access (“OFDM/OFDMA”) technique capable of easily adopting interference avoidance is applied to a cellular domain.
FIG. 1 illustrates inter-cell interference. For convenience for illustration, two base stations BS1 and BS2 and three terminals MS1, MS2 and MS3 are shown in FIG. 1.
Referring to FIG. 1, a first terminal MS1 is located at a center of a first cell C1 and is in communication with a first base station BS1. The second terminal MS2 is located in the boundary of the first cell C1 that is adjacent to a second base station BS2, and is in communication with the first base station BS1. A third terminal MS3 is located in the boundary of the second cell C2 that is adjacent to the first base station BS1, and is in communication with the second base station BS2.
Signals forwarded from the third terminal MS3 to the first base station BS1 act as interference to the first base station BS1. When the first terminal MS1 and the third terminal MS3 use the same frequency for communication, a signal to interference ratio (SIR) for an up-link signal transmitted from the first terminal MS1 to the first base station BS1 is lowered due to interference by the third terminal MS3. However, because the first terminal MS1 and the first base station BS1 are very close to each other while the third terminal MS3 and the first base station BS1 are apart from each other, the interference is not great and the first terminal MS1 is capable of performing normal communication with the first base station BS1 despite the presence of interference by the third terminal MS3. That is, the first terminal MS1 and the third terminal MS3 are allowed to use the same frequency.
On the other hand, when the second terminal MS2 and the third terminal MS3 use the same frequency for communication, interference becomes very severe. That is, because there is no great difference between distance from the second terminal MS2 to the first base station BS1 and distance from the third terminal MS3 to the first base station BS1, interference by the third terminal MS3 is very strong, thereby lowering a signal to interference ratio of an up-link signal transmitted from the second terminal MS2 to the first base station BS1. Thus, the use of the same frequency in the second terminal MS2 and the third terminal MS3 leads to interference therebetween, which makes it difficult to perform communication.
To solve these problems, a method has been proposed in which a cell is divided into a boundary portion and a center portion for assignment of tone set. This method is disclosed in Korean Laid-open Patent Publication No. 2004-0076438.
However, the use of this method causes a problem in that only one-third of available tones are assigned to the boundary portion of the cell while two-thirds of the available tones are assigned to the center portion of the cell due to a relationship between the cell and neighboring cells. That is, the available tones cannot be used efficiently since only the one-third of the available tones are assigned to terminals located in a boundary portion of an actual cell even though the boundary portion accommodates much more terminals compared to a center portion.